Tracking Sediments From Source to Sink in the Andean Orogenic Belt and Foreland Basin System

Abstract

Integration of detrital zircon U-Pb geochronological analyses of modern rivers and ancient sedimentary basins in South America enables reconstruction of erosion and sediment routing patterns during uplift of the Andes. Provenance data for modern rivers in both erosional uplands and depositional basins define source-to-sink signatures for diverse sediment sources within distinct tectonic provinces. Andean basins are fed by three main source regions: a subduction-related Mesozoic-Cenozoic magmatic arc, a retroarc fold-thrust belt composed of chiefly Paleozoic-Mesozoic sedimentary rocks, and Precambrian crystalline basement of the South American craton. Case studies for selected segments of the Andes include regions that have undergone flat-slab subduction, accretion of oceanic terranes, and variable degrees of shortening and crustal thickening. (1) In Colombia, Ecuador, and northern Peru, modern rivers reveal distinct tectonomagmatic provinces with discrete U-Pb age populations and Hf isotopic signatures. Corresponding Cretaceous-Cenozoic foreland, hinterland, and forearc basins show a wholesale reversal in sedimentary polarity from cratonic sources to magmatic arc and thrust-belt sources during initial Andean shortening. Thereafter, advance of shortening is expressed in the stepwise introduction of new source regions in the Western/Central Cordillera and Eastern Cordillera/Subandean Zone. (2) In central Argentina, modern rivers record a range of sources spanning a magmatic arc, thin-skinned thrust belt, and basement-involved broken foreland province. Within Neogene sedimentary basins, temporal shifts in provenance record a transition from high-magnitude (>100 km) thrust-belt shortening to basement partitioning of a once-contiguous foreland basin during a transition from normal- to flat-slab subduction. (3) In southern Argentina (northern Patagonia), low-magnitude (<50 km) shortening has resulted in a foreland basin fed almost exclusively by the Andean magmatic arc, with subordinate input from long-lived foreland basement massifs. These case studies highlight the utility of unambiguous source provinces in evaluating along-strike complexities in the Andes, including variations in bulk lithology, subduction angle, net shortening, crustal thickening, and continuous versus broken foreland situations. Such conditions in Andean-type systems enable robust forward modeling of provenance data and higher-order questions regarding sediment budgets, source mixing, downslope compositional changes, and expected signatures within continental-scale drainage systems such as the Amazon River and its corresponding deep-sea fan.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019